feat(tide-table): draft tracks.json from backlog.md — 25 gigs catalogued, now 33

Eight gigs had a page on the site and no tracks.json, which is the only thing
the catalog joins on, so eight real sets were invisible to every tool that
reads the corpus. All eight had a trusted setlist anchor in backlog.md
already; nothing needed PLN's ears, only a drafter.

build_gig_tracksjson.py gains build_from_backlog() and a --backlog-slug/--md
mode. Extended rather than rewritten: the existing segments-based path is
untouched, and parse_score() was lifted out of an inline closure so both modes
share one parser instead of growing a second opinion about what a score is.

gigs_total 25 -> 33, tracks_total 81, and recorded rose 40 -> 44 because the
new gigs brought their recordings into the map with them.

What these files deliberately do NOT contain, because the alternative is
inventing facts about PLN's own shows:

  - no `section` -- none of the eight has an ear-verified movement structure
  - no start_s/end_s/duration_s and no totalDuration_s -- no recording has
    been measured for any of them; each file says so in `durationBasis`
    rather than leaving the absence to be guessed at
  - `stage: null` where the frontmatter sets no stage
  - `bpm: null` where backlog.md carried no inline annotation

Title, date and venue come from content/lives/{year}/{slug}.md only, with
venue mapped from `address` exactly as the ear-verified OPAL-26 file does.

The 13 unresolved track names are IN the files, flagged `resolved: false` with
`file: null`, not dropped. A name silently disappearing is the failure this
whole pipeline exists to prevent, and six of the thirteen are opal-2025 alone
-- that gig is 11 of 17 and is the one worth a human pass:

  mephisteuf            TOP HATS
  toplap-solstice-2024  Deck the Hall
  38c3-house-of-tea     CCC0
  ensad                 Parce qu'Elle est la
  39c3-house-of-tea     TechnOrage
  le-vortex             TechnOrage
  opal-festival-2025    The Secret, 1er Septembre Pour Elle, Chere Mireille,
                        JEROME, Oct29 Love First, Cafes du plus chaud au plus
                        froid

The three `confirm:`-flagged gigs (algolia-fdlm, algorave-lyon, ete-surprise)
are still withheld by backlog_setlists.py's own design, pending PLN
confirming their anchors.

Suite: 480 passed, 2 skipped, 0 failed.